The Strangest of Spiders

In the comments to an earlier post, I promised to write a post sometime on micro-spiders. As alluded to in that post, some of the smallest spiders are mind-bogglingly tiny - the smallest known male spider, Patu digua, reaches all of 0.37 mm in length as an adult, but at least one other species known as yet only from females could potentially have a male even smaller. If one of these spiders crawled into your ear while you were sleeping, it could probably slip into your Eustachian tubes and tap on the back of your eyeballs. But even more remarkable than their small size is the bizarre morphologies on show among the micro-spiders. And no group of micro-spiders is more bizarre than the Archaeidae.

Archaeids are a bit bigger than Patu, but still pretty small - the largest examples reach about six millimetres. The name "Archaeidae", of course, means "old", and archaeids received their name because they were first described in 1854 from fossils in Baltic amber from northern Europe. In Europe, the archaeids are long gone (they may have disappeared along with the amber forests), but nearly thirty years after their initial description living examples were found in Madagascar. They are also known from Australia, while a specimen from Cretaceous Burmese amber has been placed in a living genus from South Africa and Madagascar (Penney, 2003). A species has also been described from the Jurassic of Kazakhstan, but it is uncertain whether this species is an actual archaeid or belongs to another micro-spider family such as Pararchaeidae.

Many micro-spiders show relatively long chelicerae (the fangs and their base) relative to body size, but in Archaeidae this is taken to the extreme, as can well be seen in the photo by Jeremy Miller at the top of this post. Because the trochanter (base) of the chelicerae is a rigid structure, lengthening them in spiders requires that the carapace as a whole be raised, otherwise the fangs would not be able to get anywhere near the mouth. Archaeids have developed a long "neck" supporting the eyes and chelicerae. The distinct shape of the cephalothorax together with the long chelicerae gives them an unmistakeable profile, and one common name used for the group is "pelican spiders". Despite their small size, archaeids are active hunters and voracious exclusive predators of other spiders (another common name is "assassin spiders"). It has been suggested that the lengthened chelicerae are directly related to their araneophagous diet, allowing them to strike their prey without getting too close, but as I already noted archaeids are not the only small spiders with lengthened chelicerae (though they are still the most dramatic), and I'd be interested to know if there is a correlation between small size and long chelicerae.

I'd also like to share this diagram from Wood et al. (2007) showing a molecular-derived phylogeny of the endemic Madagascan genus Eriauchenius. As can be seen, there is a fair amount of variation in the thickness of the "neck" (the darkness of the bars reflects the mean carapace height/length ratio for whichever group they subtend), and it had been suggested that those species with a particularly slender neck formed a derived clade. Wood et al. (2007) found that this does not appear to be the case, with at least two extreme narrow-neck groups - E. workmani in one and E. gracilicollis and E. lavatenda in the other - at quite divergent points in the tree. I also looks to me like at least one group - E. tsingyensis and its allies - may have gone the other way. To paraphrase a Rocky Horror Picture Show audience member - that spider has no neck.

10 comments:

OK, those are seriously weird-looking animals. Thanks for putting this post up - I had no idea.

I'm a little confused on the anatomical details, though. Where is the mouth of these spiders? If the chelicerae are long and relatively unflexible, how does the spider feed? Does it suck fluids through the chelicerae? Does it fold the chelicerae to bring the body of the prey close to the mouth?

Actually, come to think of it, I know shamefully little about feeding in Chelicerates in general. Do most feed by suction through a tube? Are there chelicerates with chewing mouthparts, a la orthopteran insects?

The mouth of a spider is normally just behind the base of the chelicerae, in front of the pedipalps. In the archaeids, it is only the attachment point of the chelicerae that is moved upwards, so the mouth is still on the underside of the animal. If the long chelicerae are lain flat along the "neck" (as one is doing in the photo at the top of the post), then they end about where the mouth is. The chelicerae are composed of the basal segment with the fang at the distal end, and the fang is able to fold back onto the basal segment, so it can hold prey as well as inject venom. The mouth itself has a labium ("lower lip") and two maxillae on either side that are between them and the fangs capable of a certain degree of food manipulation and chewing, but the spider does not chew in order to break apart the prey so much as to break it open enough to flood it with digestive juices that dissolve the prey's insides so the spider can suck them out.

The vast majority of chelicerates are liquid feeders - the only chelicerates that take particulate food are horseshoe crabs, harvestmen and solpugs (sunspiders or wind scorpions).

Aydin, I have no idea what exactly Patu eats. It belongs to a cluster of micro-spiders called symphytognathids (sensu lato) that includes a few families of tiny spiders that may or may not be closely related to each other. Most symphytognathids s.l. seem to be web-builders (members of the family Anapidae actually build tiny orb-webs all of about five millimetres across) and catch minute arthropods such as springtails.

Wow. "If one of these spiders crawled into your ear while you were sleeping, it could probably slip into your Eustachian tubes and tap on the back of your eyeballs."Thanks Chris for revisiting an old (if irrational) fear. ;}

seriously though I have loved these spiders shape ever since I first saw them...I guess for me the weirder and more unusual it is the cooler it is. I love how the moth is so far from the eyes. Reminds me of some of the long drawn out faces I used to draw in caricature, only even more extreme.

Olga, I have to admit that your question was something that I'd never actually thought about before. After a brief discussion with a colleague, we agreed that the first possibility that came to mind was that it allowed greater flexibility - a smaller waist makes it easier for the (cephalo)thorax and abdomen to be bent at different angles. This is mere speculation on our part, though.

Christopher, thank you for the reply. It's as I speculated myself, but it still doesn't convince me entirely though. Aren't spiders mostly knid web with their legs? Some species have such wide abdomen with spines, I wonder how they can move it at all.

Spiders do use their legs to manipulate silk, but the silk actually comes out from the underside of the very back-end of the abdomen. The presence of spines on the abdomen is not really an issue here, because the spines are invariably dorsal or lateral rather than ventral, so don't interfere in bringing the venters of the prosoma and opisthosoma closer together.

I'm not sure it's so straightforward for Hymenoptera, though. My colleague (who works on ants) related the need for increased flexibility to the oviposition behaviour of Hymenoptera. Again, this is speculation, and just because the waist is useful to modern arthropods in improving flexibility does not necessarily mean that that is why it first appeared.

The Author

I'm an entomologist working on the identification of terrestrial invertebrates; this site is mostly put together over my lunch break. If you'd like to comment, I can be e-mailed at gerarus at westnet.com.au.Subscribe to Catalogue of Organisms